The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known powertrain architectures include torque-generative devices, including internal combustion engines and electric machines, which transmit torque through a transmission device to an output member. One exemplary powertrain includes a two-mode, compound-split, electro-mechanical transmission which utilizes an input member for receiving motive torque from a prime mover power source, preferably an internal combustion engine, and an output member. The output member can be operatively connected to a driveline for a motor vehicle for transmitting tractive torque thereto. Electric machines, operative as motors or generators, generate an input torque to the transmission, independently of an input torque from the internal combustion engine. The electric machines may transform vehicle kinetic energy, transmitted through the vehicle driveline, to electrical energy that is storable in an electrical energy storage device. A control system monitors various inputs from the vehicle and the operator and provides operational control of the powertrain, including controlling transmission operating range state and gear shifting, controlling the torque-generative devices, and regulating the electrical power interchange among the electrical energy storage device and the electric machines to manage outputs of the transmission, including torque and rotational speed.
Transmissions within a hybrid powertrain, as described above, serve a number of functions by transmitting and manipulating torque in order to provide torque to an output member. In order to serve the particular function required, the transmission selects between a number of operating range states or configurations internal to the transmission defining the transfer of torque through the transmission. Known transmissions utilize operating range states including fixed gear states or states with a defined gear ratio. For example, a transmission can utilize four sequentially arranged fixed gear states and allow selection between the four gear states in order to provide output torque through a wide range of output member speeds. Additively or alternatively, known transmissions also allow for continuously variable operating range states or mode states, enabled for instance through the use of a planetary gear set, wherein the gear ratio provided by the transmission can be varied across a range in order to modulate the output speed and output torque provided by a particular set of inputs. Additionally, transmissions can operate in a neutral state, ceasing all torque from being transmitted through the transmission. Additionally, transmissions can operate in a reverse mode, accepting input torque in a particular rotational direction used for normal forward operation and reversing the direction of rotation of the output member. Through selection of different operating range states, transmissions can provide a range of outputs for a given input.
Operation of the above devices within a hybrid powertrain vehicle require management of numerous torque bearing shafts or devices representing connections to the above mentioned engine, electrical machines, and driveline. Input torque from the engine and input torque from the electric machine or electric machines can be applied individually or cooperatively to provide output torque. However, changes in output torque required from the transmission, for instance, due to a change in operator pedal position or due to an operating range state shift, must be handled smoothly. Particularly difficult to manage are input torques, applied simultaneously to a transmission, with different reaction times to a control input. Based upon a single control input, the various devices can change respective input torques at different times, causing increased abrupt changes to the overall torque applied through the transmission. Abrupt or uncoordinated changes to the various input torques transmitted through a transmission can cause a perceptible change in acceleration or jerk in the vehicle, which can adversely affect vehicle drivability.
Various control schemes and operational connections between the various aforementioned components of the hybrid drive system are known, and the control system must be able to engage to and disengage the various components from the transmission in order to perform the functions of the hybrid powertrain system. Engagement and disengagement are known to be accomplished within the transmission by employing selectively operable clutches. Clutches are devices well known in the art for engaging and disengaging shafts including the management of rotational velocity and torque differences between the shafts. Engagement or locking, disengagement or unlocking, operation while engaged or locked operation, and operation while disengaged or unlocked operation are all clutch states that must be managed in order for the vehicle to operate properly and smoothly.
Clutches are known in a variety of designs and control methods. One known type of clutch is a mechanical clutch operating by separating or joining two connective surfaces, for instance, clutch plates, operating, when joined, to apply frictional torque to each other. One control method for operating such a mechanical clutch includes applying a hydraulic control system implementing fluidic pressures transmitted through hydraulic lines to exert or release clamping force between the two connective surfaces. Operated thusly, the clutch is not operated in a binary manner, but rather is capable of a range of engagement states, from fully disengaged, to synchronized but not engaged, to engaged but with only minimal clamping force, to engaged with some maximum clamping force. Clamping force applied to the clutch determines how much reactive torque the clutch can carry before the clutch slips. Variable control of clutches through modulation of clamping force allows for transition between locked and unlocked states and further allows for managing slip in a locked transmission. In addition, the maximum clamping force capable of being applied by the hydraulic lines can also vary with vehicle operating states and can be modulated based upon control strategies.
Transition from one operating state range to another operating state range involves transitioning at least one clutch state. An exemplary transition from one fixed gear state to another involves unloading a first clutch, transitioning through a freewheeling, wherein no clutches remain engaged, or inertia speed phase state, wherein at least one clutch remains engaged, and subsequently loading a second clutch. A driveline connected to a locked and synchronized clutch, prior to being unloaded, is acted upon by an output torque resulting through the transmission as a result of input torques and reduction factors present in the transmission. In such a torque transmitting state, the transmission so configured during a shift is said to be in a torque phase. In a torque phase, vehicle speed and vehicle acceleration are functions of the output torque and other forces acting upon the vehicle. Unloading a clutch removes all input torque from a previously locked and synchronized clutch. As a result, any propelling force previously applied to the output torque through that clutch is quickly reduced to zero. In one exemplary configuration, another clutch remains engaged and transmitting torque to the output. In such a configuration, the transmission is in an inertia speed phase. As the second clutch to be loaded is synchronized and loaded, the transmission again enters a torque phase, wherein vehicle speed and vehicle acceleration are functions of the output torque and other forces acting upon the vehicle. While output torque changes or interruptions due to clutch unloading and loading are a normal part of transmission operating range state shifts, orderly management of the output torque changes reduces the impact of the shifts to drivability.
Slip, or relative rotational movement between the connective surfaces of the clutch when the clutch connective surfaces are intended to be synchronized and locked, occurs whenever reactive torque transmitted through the clutch exceeds actual torque capacity created by applied clamping force. Slip in a transmission results in unintended loss of torque control within the transmission, results in loss of engine speed control and electric machine speed control caused by a sudden change in back-torque from the transmission, and results in sudden changes to vehicle acceleration, creating adverse affects to drivability. Therefore, clutch transitions are known to include control measures to reduce or eliminate the occurrence of clutch slip during torque phases including during transitional locking and unlocking states.
Input torques, as described above, can originate from a number of hybrid powertrain components simultaneously. Clutches, in order to avoid slip, remain synchronized and locked with a minimum clutch torque capacity whenever reactive torque is transmitted through the clutch. Clutch torque capacity is a function of hydraulic pressure applied to the clutch. Greater hydraulic pressure in the clutch results in a greater clamping force within the clutch and a resulting higher clutch torque capacity. Because output acceleration throughout powertrain operation is a function of output torque, the various input torques, acting through the transmission to create output torque, directly impact output acceleration. Minimizing an impact upon output acceleration throughout clutch operation, including transmission operating range state shifts, can therefore be benefited by an orderly coordination of input torques resulting from various hybrid powertrain components.
The hydraulic control system, as described above, utilizes lines charged with hydraulic oil to selectively activate clutches within the transmission. However, the hydraulic control system is also known to perform a number of other functions in a hybrid powertrain. For example, an electric machine utilized within a hybrid powertrain generates heat. Known embodiments utilize hydraulic oil from the hydraulic control system in a continuous flow to cool the electric machine in a base machine cooling function. Other known embodiments additionally are known to react to higher electric machine temperatures with a selectable or temperature driven active machine cooling function, providing additional cooling in the high temperature condition. Additionally, known embodiments utilize hydraulic oil to lubricate mechanical devices, such as bearings. Also, hydraulic circuits are known to include some level of internal leakage.
Hydraulic oil is known to be pressurized within a hydraulic control system with a pump. The pump can be electrically powered or preferably mechanically driven. In addition to this first main hydraulic pump, hydraulic control systems are known to also include an auxiliary hydraulic pump. The internal impelling mechanism rotates operates at some speed, drawing hydraulic oil from a return line and pressurizing the hydraulic control system. The supply of hydraulic flow by the pump or pumps is affected by the speed of the pumps, the back pressure exerted by the hydraulic line pressure (PLINE), and the temperature of the hydraulic oil (TOIL).
The resulting or net PLINE within the hydraulic control system is impacted by a number of factors. FIG. 1 schematically illustrates a model of factors impacting hydraulic flow in an exemplary hydraulic control system, in accordance with the present disclosure. As one having ordinary skill in the art will appreciate, conservation of mass explains that, in steady state, flow entering a system must equal the flow exiting from that system. As applied to FIG. 1, a flow of hydraulic oil is supplied to the hydraulic control system by the pumps. The flow exits the hydraulic control system through the various functions served by the hydraulic control system. This exemplary embodiment includes the following functions: hydraulic oil fills clutch mechanisms in order to provide clamping force required to lock the clutch, as described above; hydraulic oil provides both base cooling and active cooling of the electric machines and other components as required; hydraulic oil is used to lubricate portions of the transmission; and hydraulic oil flows through leakage internal to the hydraulic circuit. PLINE describes the resulting charge of hydraulic oil maintained in the system: for any flow through a system, the resulting pressure within the system depends upon the flow resistance within the system. Higher flow resistance in the system results in higher system pressures for a given flow. Conversely, lower flow resistance in the system results in lower system pressures for a given flow. Applied to FIG. 1, PLINE or the pressure within the hydraulic control system, changes depending upon usage of the hydraulic control system. For example, filling a previously unfilled transmission clutch consumes a significant amount of hydraulic oil from the hydraulic control system. The orifice leading to the clutch includes low resistance in order to draw the significant amount of hydraulic oil over a short time span. As a result, during the clutch filling process, PLINE in an otherwise unchanged hydraulic control system will reduce. Conversely, for a given set of functions served by the hydraulic control system, PLINE varies based upon the flow supplied by the pumps. For any given set of flow restrictions associated with the functions served, increased flow from the pumps will result in higher PLINE. By monitoring PLINE and modulating the operation of the pump or pumps supplying hydraulic flow to the hydraulic control system, PLINE can be controlled in light of desired line pressures and changing usage of the hydraulic control system.
As described above, clutch torque capacity is a function of hydraulic pressure applied to the clutch. Minimum required clutch torque capacity depends upon the reactive torque to be transmitted through the clutch. A method to accurately control clutch torque capacity in a hybrid powertrain would be beneficial to smooth operation of the powertrain.